Laser Induced Breakdown Spectroscopy (LIBS) Apparatus Based on High Repetition Rate Pulsed Laser

ABSTRACT

This invention discloses a laser induced breakdown spectroscopy (LIBS) apparatus based on a high repetition rate pulsed laser. The laser produces a train of laser pulses at a high repetition rate in the kHz or even higher range. When the laser beam hits the sample, it generates several thousands of micro-plasmas per second. Synchronized miniature CCD array optical spectrometer modules collect the LIBS signal from these micro-plasmas. By adjusting the integration time of the spectrometer to cover a plurality of periods of the laser pulse train, the spectrometer integrates the LIBS signal produced by this plurality of laser pulses. Hence the intensity of the obtained LIBS spectrum can be greatly improved to increase the signal-to-noise ratio (SNR) and lower the limit of detection (LOD). In addition, the influence of pulse to pulse variation of the laser is minimized since the obtained LIBS spectrum is the spectrum of a plurality of micro-plasmas produced by a plurality of laser pulses. The high repetition rate laser also makes it possible for fast scanning the laser beam over the sample surface such that an average spectrum of the sample is collected to overcome the sample non-uniformity issue or for performing spectral imaging of the sample by correlating the obtained LIBS spectrum with the position of the scanning laser beam.

REFERENCE TO RELATED APPLICATION

This application claims inventions which were disclosed in ProvisionalPatent Application No. 62/046,638, filed Sep. 5, 2014, entitled “LASERINDUCED BREAKDOWN SPECTROSCOPY (LIBS) APPARATUS BASED ON HIGH REPETITIONRATE PULSED LASER” and Provisional Patent Application No. 62/050,254,filed Sep. 15, 2014, entitled “LASER INDUCED BREAKDOWN SPECTROSCOPY(LIBS) APPARATUS WITH MULTI-PURPOSE AIR CURTAIN”. The benefit under 35USC §119(e) of the above mentioned United States ProvisionalApplications is hereby claimed, and the aforementioned applications arehereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to a laser induced breakdownspectroscopy (LIBS) apparatus, and more specifically to a laser inducedbreakdown spectroscopy (LIBS) apparatus based on high repetition ratepulsed laser.

BACKGROUND

Laser induced breakdown spectroscopy (LIBS) is a type of atomic emissionspectroscopy which uses a highly energetic laser pulse as the excitationsource. The laser pulse generates a high temperature micro-plasma on thesurface of the sample. After this excitation, light that ischaracteristic of the elemental composition of the sample is emitted andanalyzed within a spectrometer. LIBS has become a very popularanalytical method in view of some of its unique features such asapplicability to any type of sample, practically no sample preparation,remote sensing capability, and speed of analysis.

Traditional laser induced breakdown spectroscopy (LIBS) apparatus isbased on single-shot lasers or lasers with very low repetition rate of<10 Hz. These lasers suffer from pulse to pulse variation in pulseenergy, pulse width, peak power, etc., which induces instability in theintensity and duration of the produced micro-plasmas. This instabilitylimits the capability of traditional LIBS apparatus for performingquantitative analysis of the subject sample. In addition, LIBS is apoint measurement technique in which the size of the sample underanalysis is typically limited to sub-millimeter (mm) per measurementpoint. However, samples under measurement generally have various levelof non-uniformity. In order to get a consistent measurement result,multiple points on the sample surface need to be measured.Conventionally, this is achieved by mechanically moving the sample inreference to the laser beam or by scanning/deflecting the laser beamusing one-dimensional (1-D) or two-dimensional (2-D) mirrors, such asgalvanometer mirrors or MEMS mirrors. Sometimes this is combined with aservo focusing mechanism for automatically focusing the laser beam ontovarious depths on an uneven surface. For traditional LIBS systems, thisprocess is very time-consuming since they can only measure a fewmeasurement points per second due to the low repetition rate of theexcitation laser.

SUMMARY OF THE INVENTION

It is thus the goal of the present invention to provide a laser inducedbreakdown spectroscopy (LIBS) apparatus based on a high repetition ratepulsed laser. The laser produces a train of laser pulses at a highrepetition rate in the kHz or even higher range. When the laser beamhits the sample, it generates several thousands of micro-plasmas persecond. Synchronized miniature CCD array optical spectrometer modulescollect the LIBS signal from these micro-plasmas. By adjusting theintegration time of the spectrometer to cover a plurality of periods ofthe laser pulse train, the spectrometer integrates the LIBS signalproduced by this plurality of laser pulses. Hence the intensity of theobtained LIBS spectrum can be greatly improved to increase thesignal-to-noise ratio (SNR) and lower the limit of detection (LOD). Inaddition, the influence of pulse to pulse variation of the laser isminimized since the obtained LIBS spectrum is the spectrum of aplurality of micro-plasmas produced by a plurality of laser pulses. Thehigh repetition rate laser also makes it possible for fast scanning thelaser beam over the sample surface such that an average spectrum of thesample is collected to overcome the sample non-uniformity issue or forperforming spectral imaging of the sample by correlating the obtainedLIBS spectrum with the position of the scanning laser beam.

BRIEF DESCRIPTION OF THE FIGURES

The accompanying figures, where like reference numerals refer toidentical or functionally similar elements throughout the separate viewsand which together with the detailed description below are incorporatedin and form part of the specification, serve to further illustratevarious embodiments and to explain various principles and advantages allin accordance with the present invention.

FIG. 1 illustrates an exemplary embodiment of the laser inducedbreakdown spectroscopy (LIBS) apparatus based on high repetition ratepulsed laser;

FIG. 2 a is a flowchart for the high repetition rate laser based LIBSsystem; and

FIG. 2 b is a flowchart for the conventional single-shot or lowrepetition rate laser based LIBS system for comparison.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions of some of the elements inthe figures may be exaggerated relative to other elements to help toimprove understanding of embodiments of the present invention.

DETAILED DESCRIPTION

Before describing in detail embodiments that are in accordance with thepresent invention, it should be observed that the embodiments resideprimarily in combinations of method steps and apparatus componentsrelated to a laser induced breakdown spectroscopy (LIBS) apparatus basedon high repetition rate pulsed laser. Accordingly, the apparatuscomponents and method steps have been represented where appropriate byconventional symbols in the drawings, showing only those specificdetails that are pertinent to understanding the embodiments of thepresent invention so as not to obscure the disclosure with details thatwill be readily apparent to those of ordinary skill in the art havingthe benefit of the description herein.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element proceeded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

An exemplary embodiment of the laser induced breakdown spectroscopy(LIBS) apparatus is shown in FIG. 1. The LIBS apparatus comprises apulsed laser 100 as the excitation light source. The pulsed laser 100 isa passively or actively Q-switched laser, or a mode-locked laser, ormore preferably a passively Q-switched diode pumped solid state (DPSS)laser, which is capable of producing a train of laser pulses at a highrepetition rate of >100 Hz, more preferably >1000 Hz (1 KHz). The pulsewidth of the laser is preferably less than 10 nanoseconds (ns), and morepreferably less than 1 nanosecond (ns). The laser beam 102 from thepulsed laser 100 is focused by an objective lens 104 onto a surface ofthe sample 106. The laser pulse produces a plasma emission, i.e. LIBSsignal 108 at the surface of the sample 106, which is collected by afocusing lens 110 to be focused into a light guide 112, such as anoptical fiber or fiber bundle. The light guide 112 then delivers theLIBS signal 108 into an optical spectrometer device 114 for spectralanalysis. The LIBS apparatus further comprises an optical window 116 infront of the objective lens 104 to protect its optical components fromcontamination. In a slight variation of the LIBS apparatus, theobjective lens 104 and the focusing lens 110 may be replaced with othertypes of optical focusing elements, such as concave mirrors, to avoidchromatic aberration of the optical lenses. In yet another variation ofthe present embodiment, the objective lens 104 may be replaced with aBessel beam generator, such as an axicon lens or an acoustic gradientlens which offers a larger focus depth than a conventional lens. TheBessel beam generator converts the laser beam 102 from Gaussian beaminto Bessel beam, which is capable of maintaining a tight focus over arelatively long distance. This helps to overcome the reproducibilityissue of LIBS signal caused by uneven sample surface. In addition, theBessel beam is less susceptible to scattering caused by dust in theenvironment.

By adjusting the integration time of the spectrometer device 114 tocover a plurality of periods of the laser pulse train, the spectrometerdevice 114 can integrate the LIBS signal produced by a plurality oflaser pulses. Hence the intensity of the obtained LIBS spectrum can begreatly improved to increase the signal-to-noise ratio (SNR) of theobtained LIBS spectrum. This unique feature of the high repetition ratelaser based LIBS system allows it to measure trace elements with verylow concentration, hence reducing the limit of detection (LOD) of theLIBS system. The increased signal intensity also lessens the sensitivityrequirement for the optical spectrometer device 114. In addition, theenergy of individual pulses in the laser pulse train can be reduced incomparison to conventional single shot or low repetition rate laserbased LIBS system to obtain the same signal level. Hence the laser pulseis less invasive to the sample. Since the obtained LIBS spectrum is thespectrum of a plurality of micro-plasmas produced by a plurality oflaser pulses, the influence of pulse to pulse variation of the laser isalso minimized.

The high repetition rate laser enables fast scanning of the samplesurface either to minimize the influence of sample non-uniformity or toperform spectral imaging of the sample. Referring back to FIG. 1, theobjective lens 104 is mounted on a vibration motor (not shown) or othertypes of vibration device, which causes the objective lens 104 tovibrate in a direction perpendicular to the laser beam (parallel withthe sample surface). The vibration pattern can be either 1-dimentional(1-D) or 2-dimentional (2-D), which results in 1-dimentional (1-D) or2-dimentional (2-D) lateral movement of the laser beam over the samplesurface. Thus the laser beam is scanned over an area of the samplesurface to excite LIBS signal from multiple measurement points. Theoptical spectrometer device 114 operates in a continuous mode to collectthe LIBS signal from all these measurement points and obtains thecorresponding LIBS spectra. Additionally, the vibration motor may causethe objective lens 104 to vibrate in a direction parallel with the laserbeam (perpendicular to the sample surface). This vibration causes thelaser beam to be focused at different depths on the sample surface. Thusthe laser beam can produce plasma emission from at least a portion ofthe measurement points even though the sample surface is uneven.

The vibration pattern of the objective lens 104 need not to be servocontrolled in the present embodiment, which greatly simplifies theoptical and mechanical design of the system. Further, the vibrationpattern can be random or irregular in nature with a predefined maximumvibration range, causing the laser beam to move over an entire area onthe sample surface. This laser beam movement, combined with the highrepetition rate of the pulsed laser 100, allows one to collect LIBSspectra from hundreds or thousands of measurement points in just a fewseconds. By performing an averaging of these spectra with a processorunit, the spectral variation caused by sample non-uniformity can begreatly reduced. The laser beam is further focused at different depthson the sample surface if a vertical lens vibration as disclosed above isincorporated. Thus the laser beam can produce strong plasma emissionfrom at least a portion of the measurement points even though the samplesurface is uneven. This allows one to obtain high quality LIBS spectrafrom at least a portion of the measurement points for an uneven surface.A predefined sorting or mathematical post-processing of the collectedspectra may be performed with a processor unit to select the LIBSspectra from those in-focus measurement points, thus obtaining a morereproducible and accurate result. In comparison with the servo focusingmechanism used in conventional LIBS system, the above disclosedtechnique does not require any complicated feedback control, hencegreatly simplifies the optical and mechanical design of the LIBS system.In addition, this variation in focus depth may be utilized for revealingadditional material information related to the changing plasma emissiondue to laser power density variation as result of focus depth change. Ina slight variation of the present embodiment, the vibration range (inboth lateral and vertical directions) of the objective lens 104 and theenergy level of the laser pulse can be feedback controlled in accordanceto the quality (e.g. the signal-to-noise ratio) of the collected theLIBS spectra to obtain the optimum measurement result. For example, thevibration range of the objective lens 104 in the vertical direction canbe feedback controlled in accordance to the number of collected LIBSspectra which are ‘in-focus’ to the surface of the sample. The vibrationrange of the objective lens 104 in the lateral direction can be feedbackcontrolled in accordance to the variation of the collected LIBS spectra,which reflects the uniformity of the sample. The energy level of thelaser pulse can be feedback controlled in accordance to the intensity orsignal-to-noise ratio of the collected LIBS spectra.

Referring back to FIG. 1, the position of the laser beam on the samplesurface may be monitored and recorded by a camera device 118. Theposition information is then correlated to the obtained LIBS spectrum ofthe corresponding measurement point to construct a two dimensional (2-D)spectral mapping of the sample surface.

As another feature of the present invention, the LIBS apparatus isequipped with an air curtain to prevent or reduce contaminant depositiononto its optical components. As disclosed above, the LIBS laser ablatesa small amount of material from the sample. This material may depositonto the optical components, such as the optical window 116 and theobjective lens 104, of the LIBS apparatus. Continued accumulation ofthese materials will contaminate the optical components and reduce boththe intensity of the laser pulse and the intensity of the collected LIBSsignal. This problem is solved by introducing an air curtain between theoptical window 116 and the surface of the sample 106. Referring to FIG.1, the air curtain is produced by a forced air flow generator, e.g. afan 120 with a pre-filtering system 122. External air passing throughthe pre-filtering system 122 is filtered and then accelerated by the fan120 to generate a forced air flow between the sample 106 and the exitoptics, i.e. the objective lens 104 and the optical window 116 of theLIBS apparatus. The forced air flow acts as an air curtain forpreventing or reducing the laser-ablated material from deposition ontothe exit optics of the LIBS apparatus. It can also be used as an air gunfor pre-cleaning the sample surface or for cooling down the temperatureof the measurement area. Further, the pre-filtering system 122 mayfilter and extract nitrogen gas from the atmosphere. The nitrogen gas issupplied to the sample surface as purge gas for increasing the LIBSsignal intensity in the ultraviolet (UV) wavelength region and enhancingthe LIBS signal for certain elements, e.g. carbon and sulfur, which havestrong spectral lines in the UV region. In a slight variation of thepresent embodiment, the pre-filtering system 122 may be combined with aninert gas generation system or a compressed gas system to supply inertgas (e.g. argon, helium) to the sample surface. The inert gas will helpincrease the intensity of the spectral lines in the deep UV region,which may be re-absorbed by the air if the LIBS measurement is performedunder open air condition.

A comparison of the presently disclosed high repetition rate laser basedLIBS system and conventional single-shot laser based LIBS system isshown in FIG. 2. In conventional single-shot laser based LIBS system asshown in FIG. 2 b, the LIBS laser produces one laser pulse at a time.The laser beam is focused onto the sample by providing servo-control tothe focusing element. The laser beam then excites plasma emission fromone measurement point on the sample. The optical spectrum of the plasmaemission is measured to obtain the LIBS spectrum of the measurementpoint. The laser beam is then moved to another measurement point tomeasure the LIBS spectrum thereof. In the high repetition rate laserbased LIBS system as shown in FIG. 2 a, the LIBS laser operatescontinuously, producing hundreds or thousands of laser pulses persecond. The laser beam is focused by a focusing element, which isvibrated by a vibration device to scan the laser beam over an area ofthe sample to excite LIBS signal from multiple measurement points. Inaddition, the focusing element may be vibrated in a directionperpendicular to the sample surface. This vibration causes the laserbeam to be focused at different depths on the sample surface, thusproducing plasma emission from at least a portion of the measurementpoints even though the sample surface is uneven. The spectrometer devicecollects LIBS spectrum continuously from the plurality of measurementpoints. A predefined sorting or other kinds of mathematicalpost-processing of the collected spectra is performed with a processorunit to select the LIBS spectra with the best quality (e.g. bestsignal-to-noise ratio). As an option, the quality (e.g. signal-to-noiseratio) of the collected LIBS spectra may be utilized to provide feedbackcontrol to the vibration device and the high repetition rate laser inorder to obtain the optimum measurement result. The high repetition ratelaser based LIBS system is capable of measuring the LIBS spectra ofmultiple measurement points in a very short period of time. This help tosolve the sample non-uniformity issue. In addition, by integrating theLIBS signal produced by a plurality of laser pulses, the intensity ofthe obtained LIBS spectrum can be greatly improved, making it possibleto measure trace elements with very low concentration. Further, theenergy level of individual laser pulses in the laser pulse train can bereduced to make the laser pulse less invasive to the sample.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential features or elements of any or all the claims.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

What is claimed is:
 1. A laser induced breakdown spectroscopy (LIBS)apparatus for measuring the LIBS spectrum of a subject, the LIBSapparatus comprising: a high repetition rate pulsed laser light sourceconfigured to produce a laser beam in the form of a plurality of laserpluses at a high repetition rate; an optical focusing element configuredto focus the laser beam onto a surface of the subject, wherein theplurality of laser pluses produce a plurality of plasma emissions fromthe surface of the subject; an optical spectrometer device configured tomeasure an optical spectrum of the plurality of plasma emissions andobtain a LIBS spectrum.
 2. The laser induced breakdown spectroscopy(LIBS) apparatus of claim 1, wherein the optical spectrometer device isset to an integration time which covers a plurality of periods of thehigh repetition rate laser pulses.
 3. The laser induced breakdownspectroscopy (LIBS) apparatus of claim 1, wherein the repetition rate ofthe laser pulse is greater than 100 Hz.
 4. The laser induced breakdownspectroscopy (LIBS) apparatus of claim 1, wherein the repetition rate ofthe laser pulse is greater than 1000 Hz.
 5. The laser induced breakdownspectroscopy (LIBS) apparatus of claim 1, wherein the laser pulses havea pulse width of less than 10 nanoseconds (ns).
 6. The laser inducedbreakdown spectroscopy (LIBS) apparatus of claim 1, wherein the laserpulses have a pulse width of less than 1 nanosecond (ns).
 7. The laserinduced breakdown spectroscopy (LIBS) apparatus of claim 1, wherein thepulsed laser light source is a Q-switched laser.
 8. The laser inducedbreakdown spectroscopy (LIBS) apparatus of claim 1, wherein the pulsedlaser light source is a mode locked laser.
 9. The laser inducedbreakdown spectroscopy (LIBS) apparatus of claim 1, wherein the pulsedlaser light source is a passively Q-switched diode pumped solid state(DPSS) laser.
 10. The laser induced breakdown spectroscopy (LIBS)apparatus of claim 1, wherein the optical focusing element comprises anoptical lens.
 11. The laser induced breakdown spectroscopy (LIBS)apparatus of claim 1, wherein the optical focusing element comprises anoptical mirror.
 12. The laser induced breakdown spectroscopy (LIBS)apparatus of claim 1, wherein the optical focusing element comprises aBessel beam generator.
 13. The laser induced breakdown spectroscopy(LIBS) apparatus of claim 1, further comprising a vibration deviceconfigured to vibrate the optical focusing element to scan the laserbeam over an area on the surface of the subject, wherein the laser beamproduces a plurality of plasma emissions from a plurality of measurementpoints on the surface of the subject and the optical spectrometer devicemeasures the plurality of plasma emissions to obtain a plurality of LIBSspectra.
 14. The laser induced breakdown spectroscopy (LIBS) apparatusof claim 13, wherein the vibration device vibrates the optical focusingelement in a direction parallel with the surface of the subject.
 15. Thelaser induced breakdown spectroscopy (LIBS) apparatus of claim 13,wherein the vibration device vibrates the optical focusing element in adirection perpendicular to the surface of the subject.
 16. The laserinduced breakdown spectroscopy (LIBS) apparatus of claim 13, wherein thevibration device produces a 1-dimentional (1-D) vibration pattern. 17.The laser induced breakdown spectroscopy (LIBS) apparatus of claim 13,wherein the vibration device produces a 2-dimentional (2-D) vibrationpattern.
 18. The laser induced breakdown spectroscopy (LIBS) apparatusof claim 13, wherein the vibration device produces a 3-dimentional (3-D)vibration pattern.
 19. The laser induced breakdown spectroscopy (LIBS)apparatus of claim 13, wherein the vibration device produces a randomvibration pattern.
 20. The laser induced breakdown spectroscopy (LIBS)apparatus of claim 13, further comprising a processor unit configured toperform a predefined sorting or mathematical post-processing of theplurality of LIBS spectra to obtain optimum LIBS spectra of the subject.21. The laser induced breakdown spectroscopy (LIBS) apparatus of claim20, wherein the processor unit provides feedback control to thevibration device based on a quality of the plurality of LIBS spectra.22. The laser induced breakdown spectroscopy (LIBS) apparatus of claim1, further comprising a camera device configured to monitor and record aposition of the laser beam on the surface of the subject.
 23. The laserinduced breakdown spectroscopy (LIBS) apparatus of claim 1, furthercomprising a forced air flow generator configured to produce a forcedair flow between the optical focusing element and the surface of thesubject.
 24. The laser induced breakdown spectroscopy (LIBS) apparatusof claim 23, further comprising an insert gas supplier for supplyinginert gas to the forced air flow generator.
 25. The laser inducedbreakdown spectroscopy (LIBS) apparatus of claim 23, wherein the insertgas supplier comprises an air-filtering system configured to extractnitrogen gas from the air.